Method for drying compact containing metal oxide, method for reducing metal oxide, and rotary-hearth-type metal reducing furnace

ABSTRACT

The present invention provides: a method for drying compacts containing water so as not to cause explosion and powdering; a method for reducing the compacts after being dried with great efficiency in a rotary-hearth-type reducing furnace; and a rotary-hearth-type metal reducing furnace. In the present invention, when compacts comprising powder containing metal oxide and carbon are dried, the critical value of a water evaporation rate, beyond which explosion occurs, is determined from the size and porosity of the compacts, then the water evaporation rate is controlled to a value not exceeding the critical value and, by so doing, the increase in the internal pressure of the compacts caused by the generation of water vapor is prevented. By the method, the explosion and cracking of the compacts are prevented. Further, when compacts are dried in a rotary-hearth-type reducing furnace, explosion is prevented by controlling the heat supply rate to the compacts through the above method and successively the compacts are incinerated and reduced in the same furnace.

TECHNICAL FIELD

The present invention relates to: a method for drying compacts producedby forming powder containing metal oxide and carbon, these beingcontained in dust, sludge and the like generated, for example, in therefining and processing of metals; a method for reducing the driedcompacts in a reducing furnace equipped with a rotary hearth; a methodfor drying and successively reducing compacts into metal in a reducingfurnace equipped with a rotary hearth; and further a metal reducingfurnace of a rotary-hearth-type.

BACKGROUND ART

There are various types of processes as processes for producing reducediron and ferroalloy and, among those processes, rotary-hearth-typeprocesses are practically used as the processes having highproductivity. A rotary-hearth-type process is the one mainly composed ofan incinerator of the type wherein a refractory hearth having thetoroidal shape of a disc (the center of which is lacked) rotates at aconstant speed (hereunder referred to as “rotary furnace”) on railsunder fixed refractory ceiling and sidewalls and is used for reducingmetal oxide. The diameter of the hearth of a rotary furnace is 10 to 50m and the width thereof is 2 to 6 m.

Powder containing metal oxide as raw material is mixed with acarbon-type reducing agent, is thereafter formed into raw materialpellets, and is fed to a rotary furnace. An advantage of this is thatthe raw material pellets are laid close and still on the hearth andtherefore the raw material pellets hardly collapse in the furnace.Another advantage thereof is that the problem of powdered raw materialsticking to refractories is avoided and thus the bulk yield of theproduct is high. Further, this method has so far been employedincreasingly since it has a high productivity and allows a lessexpensive carbon-type reducing agent and powdered raw material to beused.

In addition, a rotary hearth process is also effective for reducingsteelmaking dust generated from a blast furnace, a converter and anelectric arc furnace and thickener sludge generated from a rolling milland removing impurities, thus is used also as a dust processing means,and is a process effective for the recycling of resources.

The operation of a rotary hearth process is outlined hereunder.

Firstly, a carbon-type reducing agent is mixed with metallic oxidecomprising raw material of ore, dust and sludge in the amount requiredfor reducing the metallic oxide and thereafter the mixture is formedinto pellets several to over ten mm in size with a granulator such as apan-type pelletizer while water is sprayed so that water content isabout 10%. When the grain sizes of raw material ore and a reducing agentare large, they are crushed with a crusher such as a ball mill andthereafter kneaded and granulated.

The pellets are fed to a rotary hearth and laid in layers. The pelletslaid on the hearth in layers are heated rapidly and incinerated for 5 to20 min. at a high temperature of about 1,300° C. During the process,metal oxide is reduced by the reducing agent. mixed in the pellets andmetal is formed. The metallization ratio after the reduction varies inaccordance with metal to be reduced, and is 95% or more in the case ofiron, nickel, or manganese and 50% or more even in the case of chromiumthat is hard to reduce. Further, when dust coming from the steelmakingindustry is processed, impurities such as zinc, lead, alkali metal,chlorine, etc. are volatilized and removed in accordance with thereducing reaction and therefore the dust may easily be recycled to ablast furnace or an electric arc furnace.

In such a method of reducing metal and steelmaking dust with a rotaryhearth, to form raw material and a reducing agent into pellets is anessential requirement. Therefore, it is important to put the mixture ofmetal oxide powder as raw material and a reducing agent in the stateliable to be granulated in a preliminary treatment of the raw materialand, for that purpose, various methods such as preliminary crushing ofraw material, kneading in a ball mill and the like are employed.

As explained above, the reduction of metal oxide by the conventionalrotary hearth method is excellent in productivity and production costsand is a method for producing metal economically. However, the problemsof the method have been that: it is important to pelletize raw materialand a reducing agent; thus it is necessary either to select raw materialhaving an excellent granulation property or to improve a granulationproperty by installing an expensive crusher and crushing raw material.This incurs a large cost.

Actually, when ore such as iron ore is used as raw material, as the sizeof the raw material ore is large, the raw material has generally beencrushed into several tens of microns in average diameter and thereaftergranulated and produced into pellets. For that reason, the drawbacks ofthe method have been that: the cost of equipment for a crushing processis high; electricity is required for the operation of a crusher; and themaintenance accompanying the wear of the crusher is expensive.

There are some cases where pulverized raw material is used for saving acrushing cost. However, in such a case, the selection of raw material islimited and thus it has not been a commonly adaptable method. Forsolving the problem, it is effective to use powdered ore after wetseparation, thickener dust from a blast furnace or a converter, sludgein a scale pit of a rolling process, precipitated sludge from a picklingprocess, or the like. However, in those cases too, a problem has beenthat a water content is sometimes excessive and therefore raw materialis hardly granulated. In particular, such raw material is composed ofvery fine powder one to several tens microns in size and, as a result,it is likely to be slurry in the state of containing water, or, evenafter the raw material is dehydrated with a vacuum dehydrator or afilter press, water remains by 30 to 50% and, as it is, it is difficultto granulate because too much water is contained.

One of the methods for solving the problems is to granulate powder rawmaterial after it has been dried completely with a heat source such as ahot blast. However, in this method, the powder raw material forms apseudo-agglomeration during the drying and thus it is impossible togranulate it as it is. Therefore, the pseudo-agglomeration of the powderraw material has been crushed, formed in the state of fine powder again,thereafter mixed with other material and water, then granulated and,thereafter, reduced on a rotary hearth.

As a result, when the above method is employed, good compacts can beproduced and, if the compacts are dried efficiently, metal oxide isreduced stably. However, by a conventional technology, a method fordrying such compacts, in consideration of the physical state of thecompacts, is not established sufficiently and it has merely beenconsidered that only the drying of the compacts is enough. As a result,there have been problems in that the compacts crack and powdering occursabundantly from the surfaces thereof. Furthermore, when dryingconditions are worse, the compacts have sometimes exploded. Therefore, ameans for solving the problems has been desired for a long time. Here,though a method of drying compacts beforehand is an effective means, theproblem of the method is still that the method requires a heat sourceand a device for exclusively vaporizing the water content, even afterthe compacts are dried, by consuming a large amount of heat and issomewhat disadvantageous economically.

In particular, when dust or sludge generated in the metal refiningindustry and processing industry such as the steelmaking industry iscollected from a wet dust collector or a settling tank, the dust orsludge contains a large amount of water, 90% at the most, and when it isattempted to reduce it by a rotary hearth process, the drying processand the succeeding crushing process have been problems.

As a method of using raw material without granulation in a rotary hearthprocess for solving the problems, for example, Japanese UnexaminedPatent Publication No. H11-12619 discloses the method wherein rawmaterial is formed into a tile shape with a compression molding machineand used in a rotary hearth process. Even in this method however, therehave been problems in using raw material containing a large amount ofwater. That is, the problems have been that, as shown in JapaneseUnexamined Patent Publication No. H11-12624: the water content in rawmaterial is required to be adjusted to 6 to 18% and for that purpose adrying process is required in addition to a preliminary dehydrationprocess; for that reason the complicated control of a water content isrequired. Further, another problem has been that, in order to chargesuch raw material, a complicated charging machine is required as shownin Japanese Unexamined Patent Publication No. H11-12621 and themaintenance cost of the equipment is high.

Further, when such a type of raw material, containing water, is directlycharged into a high temperature rotary furnace, the problems have beenthat: explosion occurs due to a high water content in accordance withthe evaporation of water; the raw material is pulverized and taken awaywith an exhaust gas; and thus the product yield deteriorates extremely.In a rotary hearth process, the temperature in the furnace is generallythe lowest in the vicinity of a raw material inlet and about 1,150° C.to 1,200° C. even there. At such a high temperature, compacts in a wetstate entail the problem of explosion accompanying sudden waterevaporation. Even when an explosion does not arise, exfoliation occursat the corner portions and the surface due to the eruption of watervapor. Therefore, even though reduction operation can be carried out,there have been the problems in that the bulk ratio of the reducedproduct decreases and the ratio of powder generated from the compactsincreases. As a result, there remain the problems in that the ratio ofpowder metal that is lost in an exhaust gas increases relatively and theyield deteriorates.

The object of the present invention is to provide: a method for dryingefficiently compacts comprising powder raw material containing waterwithout the generation of explosion or cracking; a method for reducingcompacts that makes it possible to reduce the compacts at a high yieldwithout the generation of an explosion or the like even when compacts,in the state of powder containing water, are supplied directly to arotary furnace and reduced; and a rotary-hearth-type metal reducingfurnace therefor, those having not so far been realized by conventionalmethods.

DISCLOSURE OF THE INVENTION

The present invention has been established in view of the above problemsand the gist thereof is as follows:

(1) A method for drying compacts characterized by, in the event ofdrying compacts containing powder of metal oxide and carbon and alsowater in mass percentage by not less than 0.2 times the porosity inpercentage, controlling the evaporation rate of water contained in saidcompacts to not more than the value V defined below;V=300P ² /D,where, V means a critical evaporation rate of water (an evaporation rateof water per one dry mass kilogram of compacts (g/kg/sec.)), D a cuberoot of the volume of a compact (mm), and P a porosity (−).

(2) A method for drying compacts characterized by, in the event ofdrying compacts containing powder of metal oxide and carbon and alsowater in mass percentage by not less than 0.2 times the porosity inpercentage, controlling the rate of heat supply to said compacts to notmore than the value Hin defined below;Hin=820P ² /D,where, Hin means a critical heat supply rate (a heat supply rate per onedry mass kilogram of compacts (kw/kg)), D a cube root of the volume of acompact (mm), and P a porosity (−).

(3) A method for drying compacts according to the item (1),characterized by, in the event of drying compacts having a cube root ofthe volume of 5 to 21 mm and a porosity of 22 to 32% from the statewherein 4.4 mass % or more of water is contained in said compacts,controlling the evaporation rate of water in said compacts to not morethan 0.7 g/sec. per one dry mass kilogram of compacts.

(4) A method for drying compacts according to the item (2),characterized by, in the event of drying compacts having a cube root ofthe volume of 5 to 21 mm and a porosity of 22 to 32% from the statewherein 4.4 mass % or more of water is contained in said compacts,controlling the rate of heat supply to said compacts to not more than1.9 kw per one dry mass kilogram of compacts.

(5) A method for drying compacts according to the item (1),characterized by, in the event of drying compacts having a cube root ofthe volume of 5 to 21 mm and a porosity of more than 32 to 40% from thestate wherein 6.4 mass % or more of water is contained in said compacts,controlling the evaporation rate of water in said compacts to not morethan 1.3 g/sec. per one dry mass kilogram of compacts.

(6) A method for drying compacts according to the item (2),characterized by, in the event of drying compacts having a cube root ofthe volume of 5 to 21 mm and a porosity of more than 32 to 40% from thestate wherein 6.4 mass % or more of water is contained in said compacts,controlling the rate of heat supply to said compacts to not more than3.5 kw per one dry mass kilogram of compacts.

(7) A method for drying compacts according to the item (1),characterized by, in the event of drying compacts having a cube root ofthe volume of 5 to 21 mm and a porosity of more than 40 to 55% from thestate wherein 8 mass % or more of water is contained in said compacts,controlling the evaporation rate of water in said compacts to not morethan 2.3 g/sec. per one dry mass kilogram of compacts.

(8) A method for drying compacts according to the item (2),characterized by, in the event of drying compacts having a cube root ofthe volume of 5 to 21 mm and a porosity being more than 40 to 55% fromthe state wherein 8 mass % or more of water is contained in saidcompacts, controlling the rate of heat supply to said compacts to notmore than 6.2 kw per one dry mass kilogram of compacts.

(9) A method for drying compacts according to any one of the items (1)to (8), characterized by using powder containing metallic oxide derivedfrom a metal producing process and carbon individually or in mixture assaid powder of metal oxide and carbon.

(10) A method for reducing metal oxide characterized by incinerating andreducing compacts dried by a method according to any one of the items(1) to (8) at 1,100° C. or higher in a rotary-hearth-type furnacewherein compacts containing powder of metal oxide and carbon are loadedon the upper surface of a rotating toroidal hearth and incinerated andreduced by gas combustion heat at the upper inner space of the furnace.

(11) A method for reducing metal oxide characterized by incinerating andreducing compacts dried by a method according to the item (9) at 1,100°C. or higher in a rotary-hearth-type furnace wherein compacts containingpowder of metal oxide and carbon are loaded on the upper surface of arotating toroidal hearth and incinerated and reduced by gas combustionheat at the upper inner space of the furnace.

(12) A method for reducing metal oxide characterized by, after dryingcompacts by a method according to any one of the items (1) to (8) in arotary-hearth-type furnace wherein compacts containing powder of metaloxide and carbon are loaded on the upper surface of a rotating toroidalhearth and incinerated and reduced by gas combustion heat at the upperinner space of the furnace, successively incinerating and reducing saidcompacts at 1,100° C. or higher in said furnace.

(13) A method for reducing metal oxide characterized by, after dryingcompacts by a method according to the item (9) in a rotary-hearth-typefurnace wherein compacts containing powder of metal oxide and carbon areloaded on the upper surface of a rotating toroidal hearth andincinerated and reduced by gas combustion heat at the upper inner spaceof the furnace, successively incinerating and reducing said compacts at1,100° C. or higher in said furnace.

(14) A rotary-hearth-type metal reducing furnace, wherein compactscontaining powder of metal oxide and carbon are loaded on the uppersurface of a rotating toroidal hearth and incinerated and reduced by gascombustion heat at the upper inner space of the furnace and the reducedcompacts are discharged, characterized in that the area of the rotaryfurnace from the portion where the raw material powder compacts aresupplied to a portion 30 to 130 degrees apart from said portion in therotation direction is used as the drying zone of the compacts.

(15) A rotary-hearth-type metal reducing furnace according to the item(14), characterized in that: an exhaust gas flue is installed at aportion 30 to 130 degrees apart from the portion where the raw materialcompacts are supplied in the rotation direction; and the area from theportion where the raw material compacts are supplied to the portionwhere the exhaust gas flue is installed is used as the drying zone.

(16) A rotary-hearth-type metal reducing furnace according to the item(14), characterized in that: a partition having a gap between the lowerend thereof and the rotary hearth is installed at a portion 30 to 130degrees apart from the portion where the raw material compacts aresupplied in the rotation direction; and the area from the portion wherethe raw material compacts are supplied to the portion where thepartition is installed is used as the drying zone.

(17) A rotary-hearth-type metal reducing furnace according to any one ofthe items (14) to (16), characterized by being equipped with a means forcooling the hearth between the portion where the reduced compacts aredischarged and the portion where the raw material compacts are supplied.

(18) A rotary-hearth-type metal reducing furnace according to any one ofthe items (14) to (16), characterized by being equipped with watercooling means on the ceiling and parts of the sidewalls between theportion where the reduced compacts are discharged and the drying zone inthe furnace.

(19) A rotary-hearth-type oxidizing metal reducing furnace according toany one of the items (14) to (16), characterized by being equipped withheating burners on the sidewalls of said drying zone.

(20) A rotary-hearth-type metal reducing furnace according to any one ofthe items (14) to (16), characterized by being equipped with: a meansfor cooling the hearth between the portion where the reduced compactsare discharged and the portion where the raw material compacts aresupplied; and water cooling means on the ceiling and parts of thesidewalls between the portion where the reduced compacts are dischargedand the drying zone in the furnace.

(21) A rotary-hearth-type metal reducing furnace according to any one ofthe items (14) to (16), characterized by being equipped with: a meansfor cooling the hearth between the portion where the reduced compactsare discharged and the portion where the raw material compacts aresupplied; and heating burners on the sidewalls of the drying zone.

(22) A rotary-hearth-type metal reducing furnace according to any one ofthe items (14) to (16), characterized by being equipped with: watercooling means on the ceiling and parts of the sidewalls between theportion where the reduced compacts are discharged and the drying zone inthe furnace; and heating burners on the sidewalls of the drying zone.

(23) A rotary-hearth-type metal reducing furnace according to any one ofthe items (14) to (16), characterized by being equipped with: a meansfor cooling the hearth between the portion where the reduced compactsare discharged and the portion where the raw material compacts aresupplied; water cooling means on the ceiling and parts of the sidewallsbetween the portion where the reduced compacts are discharged and thedrying zone in the furnace; and further heating burners on the sidewallsof said drying zone.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing an example of equipment for reducing metaloxide, comprising a forming machine, a compact dryer and arotary-hearth-type reducing furnace, to which the present invention isapplied.

FIG. 2 is a graph showing the relationship between the maximum waterevaporation rate (the critical evaporation rate) in the situation wherecompacts do not explode or the rate of powdering is 10% or less duringthe drying of the compacts and the quotient obtained by dividing thesquare of a porosity (P²) by the representative diameter D of a compact.Note that, in the figure, the unit of a critical evaporation rate isg/kg/sec. and the unit of the quotient obtained by dividing the squareof a porosity (P²) by the representative diameter D of a compact is1/mm.

FIG. 3 is a view showing the structure of a rotary-hearth-type metalreducing furnace having the function of drying compacts in the furnaceand a means for cooling the hearth and the atmosphere in the furnaceaccording to the present invention.

FIG. 4 is a view showing the structure of a rotary-hearth-type metalreducing furnace having the function of drying compacts in the furnaceaccording to the present invention.

FIG. 5 is a view showing the outline of an example of equipment forreducing metal, comprising an extrusion forming machine and arotary-hearth-type reducing furnace, to which the present invention isapplied.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 shows an example of the entire configuration of reducingequipment comprising a rotary-hearth-type reducing furnace and apreliminary treatment apparatus of metallic oxide raw material to be fedto the reducing furnace on the basis of the present invention. Thefigure shows the entire configuration of equipment that producescompacts from powder with a forming machine and reduces them in arotary-hearth-type reducing furnace. The equipment mainly comprises aforming machine 1, a compact dryer 2 and a rotary furnace 3. As aforming machine 1, any type may be employed and a pan-type pelletizer, abriquette forming machine or an extrusion forming machine is generallyused, as described later. Further, a raw material storing apparatus anda product treatment apparatus are included in the equipmentconfiguration, but they are not shown here because they are notimportant in the explanation on the method and equipment of the presentinvention. A powder containing metal oxide and carbon in the state ofcontaining water is formed into compacts with a forming machine 1 andthe formed compacts are dehydrated and dried with a compact dryer 2.Further, the dried compacts are incinerated and reduced in a rotaryfurnace 3. Note that, when compacts can withstand rapid drying or theheat load on the compacts can be reduced in the vicinity of the portionwhere the compacts are loaded in a rotary furnace 3 by theafter-mentioned method of the present invention, the compact dryer 2 maybe omitted.

The present invention is a method of properly drying compacts producedin the state of containing water, namely wet compacts, using powdercontaining metal oxide and carbon as raw material. In actual operation,a method wherein wet compacts are dried by hot blast or the like with anexclusive dryer or a method wherein wet compacts are dried at arelatively low temperature portion in a rotary furnace where a gastemperature is properly controlled is employed. In order to accomplishthe above object, the present inventors searched for conditions forproperly drying compacts comprising powder containing metal oxide andcarbon. For that purpose, the present inventors carried out theoreticalanalyses on the flow of water vapor in compacts and experiments using asmall hot blast dryer and a box type electric arc furnace.

Firstly, prior to the experiments, the present inventors carried outhydrodynamic technological analyses with respect to physical phenomenaof gas flow at the time of evaporation of water in compacts from theviewpoint of the analysis of gas flow passing through a narrow path.Secondly, the present inventors carried out the experiments of dryingactual compacts and established the treatment standard for dryingcompacts.

Firstly, on the basis of a physical model showing the relationshipbetween a flow rate and resistance of a fluid flowing in fine pores, thepresent inventors analyzed the pressure at the time when water vaporflowed among particles in compacts. From the model analysis, it has beenfound that a flow resistance per unit length when water vapor flows inpores is inversely proportional to the diameter of the paths of thepores and is proportional to the flow rate of the water vapor. Further,as a result of the observation of the compacts, it has been found thatthe path diameter of pores is almost proportional to the porosity.Furthermore, from geometric conditions of the interior of compacts, ithas been found that the flow rate of water vapor in paths isproportional to a water vapor generation rate per unit volume ofcompacts and is inversely proportional to the porosity. Here, a porosityis defined as the ratio of the volume of voids to the volume of compactsin the present invention and is generally a value obtained by dividingan apparent specific gravity of compacts by a true specific gravity ofpowder.

On the basis of the studies in consideration of hydrodynamic conditionsand geometric conditions of compacts, when the porosity in compacts isconstant, the following relational expression is established;

(Center portion pressure)=A (Path diameter)⁻¹ (Path length)(Water vaporflow rate in path), where, a path length is proportional to the diameterof compacts. Further, the expression is converted into the followingexpression from aforementioned relation;

(Center portion pressure)=B (Porosity)⁻¹ (Representative diameter){(Water vapor generation rate per mass)/(Porosity)}=B (Water vaporgeneration rate per mass)(Porosity)⁻² (Representative diameter).Furthermore, the expression is converted into the following expression;

(Water vapor generation rate per unit mass)=C (Center portionpressure)(Porosity)²/(Representative diameter), where A, B and C areconstants influenced by the physical state of compacts and the physicalproperties of a gas.

An internal pressure increases as water vapor is generated. When aninternal pressure exceeds the pressure under which compacts canwithstand, there arise the problems of explosion, cracking and powderingof the surface of the compacts. The present inventors judged dryingconditions as acceptable when neither explosion nor cracking occurred orthe ratio of powder generated from a surface was 10% or less under thedrying conditions. Here, a powdering ratio is defined by the ratio ofthe mass (mass %) of compacts less than 5 mm in size obtained whencompacts after dried are sieved with a 5 mm sieve to the mass of thecompacts before the sieving. From the results and the above expressions,a water vapor generation rate at an explosion limit was evaluatedquantitatively. The limit (critical pressure) that compacts canwithstand is a value related to the bonding strength of particles in thecompacts and the main factor of the bonding strength is the phenomenonaccompanying the physical adhesiveness among particles. The presentinventors clarified that the bonding strength of particles in compactswas almost constant when a specific binder was not used. Here, a watervapor generation rate (critical evaporation rate) at the time when acenter portion pressure reaches the critical pressure is expressed bythe following expression (a) by formulating an evaluation expression inconsideration of the aforementioned analysis results and the observationresults of the compacts and summarizing the items to be evaluated withconstant values;V=KP ² /D  (a).Further, since an evaporation rate of water is proportional to a heatsupply rate, the rate of heat supplied to compacts at a criticalpressure (critical heat supply rate) is shown by the followingexpression (b);Hin=LP ² /D  (b).In the expressions (a) and (b), V means a water vapor evaporation rateper dry mass kg of compacts at the critical pressure (g/kg/sec.), Hin aheat supply rate per dry mass kg of compacts at the critical pressure(kw/kg), P a porosity (−), and D a cube root of the volume of a compact(mm) that represents the size of the compact. Here, K and L are theconstants. In order to evaluate compacts having different shapes in thesame way, a cube root of the volume of a compact is used for theevaluation of the size of the compact and is hereunder referred to asthe representative diameter.

In order to determine the constants K and L of the expressions (a) and(b) that stipulated appropriate drying conditions, experiments werecarried out by using a heater for experiments. In the experiments, a hotblast type dryer 5 liters in volume and an electric arc furnace 10liters in volume were used. Raw material for the experiments was powderto be used in a rotary-hearth-type reducing furnace. The powder hadaverage diameters of 4 to 50 microns and contained 63 mass % iron oxideand 15 mass % carbon. The compacts used for the experiments were asfollows; {circle around (1)} spherical compacts produced with a pan-typepelletizer and having a porosity of 22 to 32%, {circle around (2)}compacts produced with a briquette forming machine and having a porosityof over 32 to 40%, and {circle around (3)} columnar compacts producedwith an extrusion forming machine and having a porosity of more than 40to 55%. Those forming methods are explained below. The compacts {circlearound (1)} were produced by rotating fine powder on a rotating disc,the compacts {circle around (2)} were formed by using a pair of rollshaving dimples on their surfaces and pushing powder into the dimpleswhile the rolls were rotating, and the compacts {circle around (3)} wereformed by pushing wet powder into a penetrating nozzle. Here, therepresentative diameters of the compacts were 5 to 21 mm.

In the experiments, the heat supply rates to the compacts were changedvariously by changing the hot blast temperature of the hot blast typedryer or the internal temperature of the box type electric arc furnace.Among the test results, the cases where explosion did not occur and thepowdering loss from the surface was 10% or less (powdering ratio was 10%or less) were judged to be good drying conditions. The upper limit of anevaporation rate in the drying under such good drying conditions wasdefined as a critical evaporation rate (namely, water amount evaporatedfor one second per dry mass kg of compacts) and the heat supply rate atthe time was defined as a critical heat supply rate. Thereafter, thosevalues were determined.

The results are shown in FIG. 2. FIG. 2 shows the relationship betweenthe quotient obtained by dividing the square of a porosity by therepresentative diameter of a compact (P²/D) and the critical evaporationrate (V). The results were subjected to a multiple regression analysisand the value K in the expression (a) was determined to be 300. Further,the value L in the expression (b) that determined a critical heat supplyrate (Hin) was 820. Here, the unit of V was g/kg/sec., the unit of Hinwas kw/kg, and P had no units.Critical evaporation rate=300P ² /D  (1),Critical heat supply rate=820P ² /D  (2).

Further, in the experiments, when a water ratio was less than 0.2 timesa porosity, explosion and powdering of a surface did not occur eventhough the values deviated from the critical values calculated from theexpressions (1) and (2) because the generated water vapor amount wassmall. Therefore, the present invention is effective in the case where awater ratio is not less than 0.2 times a porosity.

On the basis of the above analysis results, operations for dryingcompacts are carried out appropriately with actual equipment. Thecompact dryer 2 in the equipment shown in FIG. 1 is a hot blast type andthe heat supply rate to the compacts is controlled. Note that, the dryer2 may be of any type as long as the heat supply rate is controllable.The compacts containing water that have been produced by theaforementioned three methods with the forming machine 1 are supplied tothe compact dryer 2. Here, a heat supply rate not higher than thecritical heat supply rate V obtained from the expression (2) is employedin accordance with the representative diameter and the porosity of thecompacts. It is effective to adjust a heat supply rate by thetemperature and the flow rate of hot blast. The evaporation rate ofwater of the compacts at the time is controlled to a value not higherthan the critical evaporation rate obtained from the expression (1) inaccordance with the representative diameter and the porosity of thecompacts likewise.

In an actual operation by a rotary hearth process, the compacts used ina rotary furnace 3 have appropriate sizes for improving the heattransfer property in the compacts and maintaining the shapes of thecompacts and a desirable representative diameter is in the range from 5to 21 mm. The reason is as follows: when compacts are too large and inexcess of 21 mm in representative diameter, the inside heat transferbecomes slow, the reduction does not complete within the 7 to 20 min.that is an appropriate reduction time range in a rotary furnace, andcracking occurs at the time of falling; and, on the other hand, when arepresentative diameter is 5 mm or less, the compacts are too small, thecompacts have to be loaded in three to five layers for securing anappropriate amount of the compacts per floor area and, in this case theheat transfer of the compacts in the middle layers is deteriorated, thusthe reducing reaction is also deteriorated.

The compacts produced with the forming machine 1 are dried with thecompact dryer 2. In the case of dense compacts that are produced by sucha method as to use a pan-type pelletizer and have a porosity of 22 to32%, the water evaporation rate is controlled to not more than 0.7g/sec. per one dry mass kg of compacts when the compacts having therepresentative diameter of 5 to 21 mm are dried from the state ofcontaining not less than 4.4 mass % water. The water evaporation rate iswithin the critical evaporation rate obtained from the expression (1),thus in the good drying conditions, and does not cause the problems ofexplosion and powdering of the compacts. In this drying method, a heatsupply rate is controlled to not more than 1.9 kw per one dry mass kg ofcompacts. The heat supply rate is relatively low and therefore thecompacts should be dried at a relatively low temperature. A desirabledrying temperature is 400° C. or lower in the case of a hot blast typedryer.

In the case of compacts that are produced by a method using a briquetteforming machine and have a porosity of more than 32 to 40%, the waterevaporation rate of the compacts having the representative diameter of 5to 21 mm is controlled to not more than 1.3 g/sec. per one dry mass kgof compacts when the compacts are dried from the state of containing notless than 6.4 mass % water. In this drying method, the average heatsupply rate is controlled to not more than 3.5 kJ per one dry mass kg ofcompacts. In the drying of such compacts, as a somewhat higher heatsupply rate is allowed, a desirable drying temperature is in the rangefrom 200° C. to 550° C. in the case of a hot-blast-type dryer.

Further, in the case of very porous compacts that are produced by amethod using an extrusion forming machine and have a porosity of morethan 40 to 55%, the water evaporation rate of the compacts having therepresentative diameter of 5 to 21 mm is controlled to not more than 2.3g/sec. per one dry mass kg of compacts when the compacts are dried fromthe state of containing not less than 8 mass % water. In the dryingmethod, the average heat supply rate is controlled to not more than 6.3or 6.2 kw per one dry mass kg of compacts. In this drying of thecompacts, as a relatively high heat supply rate is allowable, adesirable drying temperature is in the range from 300° C. to 900° C. inthe case of a hot blast type dryer. Further, if a shorter drying time isdesired, the most appropriate drying temperature is about 800° C.

In the aforementioned case of drying compacts with a dryer, when thecompacts are loaded on a rotary furnace 3, the temperature at theportion where the compacts are supplied is high, thus explosion andpowdering caused by rapid heating are problems and, therefore, it isdesirable to control the water content of the compacts after drying tonot more than 1 mass %.

After the compacts are dried, they are fed to a rotary furnace 3. As thecompacts do not contain excessive water, the problems of explosion andpowdering do not occur even when the heating rate of the compacts ishigh in the rotary furnace 3. For example, such a high heating rate thatthe surface temperature of compacts is raised to 1,200° C. for about 3min. may be acceptable. The compacts are incinerated by being heated inthe furnace. As a result, carbon contained in the compacts functions asa reducing agent and reduces solid iron oxide and solid manganese oxide.In this case, the reduction proceeds if the maximum temperature is1,100° C. or higher, desirably 1,200° C. to 1,400° C. and, under thiscondition, the reducing reaction terminates in 7 to 15 min. The compactshaving been incinerated and reduced are discharged from the rotaryfurnace 3. Thereafter, the high temperature compacts are cooled with areduced compact cooling device not shown in FIG. 1 and reduced productsare obtained. When the reduced products are used at a high temperaturein an electric arc furnace or the like, a cooling process may be omittedin some cases.

There is a method of drying compacts in a rotary furnace 3 without theuse of a compact dryer 2. An example of the equipment configuration isthe one obtained by removing the compact dryer 2 from the equipmentconfiguration shown in FIG. 1. An example of the structure of a rotaryfurnace having such a function is shown in FIG. 3. FIG. 3 is a sectionalview of a rotary furnace 3 in the circumferential direction and showsthe structure in the vicinity of the drying zone. In the structure,compacts in a wet state are loaded on the hearth 6 in the drying zone 5with the compact feeder 4 and the compacts 12 are dried there. Thehearth 6 rotatively moves toward the right continuously and feeds thecompacts 12 having dried to the reducing zone 7. In the reducing zone 7,the compacts 12 are incinerated and reduced. In the method of dryingcompacts 12 in a furnace too, it is necessary to properly control theheat supply rate at the portion where compacts 12 are supplied so thatthe explosion of the compacts 12 and the powdering of the surfacethereof may not occur. In the drying zone 5 too, it is necessary tocontrol the water evaporation rate of compacts 12 to not more than thecritical evaporation rate (V) and further the heat supply rate to notmore than the critical heat supply rate (Hin).

In the case of compacts 12 that are produced with a pan-type pelletizer1 and have the porosity of 22 to 32% and the representative diameter of5 to 21 mm, the water evaporation rate from the state of not less than4.4 mass % water is controlled to not more than 0.7 g/sec. per one drymass kg of compacts 12 at the drying zone 5. Further, the heat supplyrate is controlled to not more than 1.9 or 1.8 kw per one dry mass kg ofcompacts 12. In this method, the compacts 12 should be dried while theportion where the compacts 12 are supplied is kept at a relatively lowtemperature. In order to reduce such dense compacts after they aredried, an exclusive dryer, as shown in the equipment configuration ofFIG. 1, is generally used for the drying. However, when such anexclusive dryer is omitted, the atmospheric temperatures at the portionsbefore and after the portion where the compacts 12 are supplied in arotary furnace 3 are lowered and wet compacts 12 are fed. When compactshave the porosity of 22 to 32%, a preferable temperature at this portionis 200° C. to 450° C.

In a rotary furnace 3, a hearth 6 of a high temperature movescontinuously to the portion where raw material compacts are supplied,namely the raw material compact supply portion and, therefore, theatmospheric temperature usually becomes about 800° C. to 1,000° C. if nomeasures are taken. Therefore, some sort of technological contrivancesare required for lowering the temperature at the raw material compactsupply portion to about 200° C. to 450° C. That is, it is necessary tocool the hearth 6 before the compacts 12 are supplied, avoid introducingexhaust gas generated during the incineration and reduction in thereducing zone 7 to the portion, and compulsorily cool the portionsbefore and after the portion where the compacts 12 are supplied. FIG. 3shows an example of equipment having the structure that absorbs radiantheat of the hearth 6 by installing the water cooling panels 9 on theceiling between the screw discharger 8 for discharging the reducedcompacts 13 and the compact feeder 4 and on the parts of the ceiling inthe drying zone 5. Further, the drying zone 5 and the reducing zone 7are separated from each other with the exhaust gas discharging flue 10so that the high temperature exhaust gas in the reducing zone 7 may notflow in. In this case, at the latter half of the drying zone 5, sinceonly the heat transferred from the hearth 6 is insufficient forsupplying heat to the compacts 12, heating burners 11 may be installedon the sidewalls and heat source for drying may be supplied from theburners.

In the case of compacts having a porosity of more than 32 to 40%, suchas the compacts produced with an aforementioned briquette formingmachine, as long as the heat supply rate is controlled to about 3.5 kwper one dry weight kg of compacts and the water evaporation rate is alsocontrolled up to 1.3 g/sec. per one dry weight kg of compacts, explosionof the compacts 12 and the powdering of the surface thereof do notoccur. The atmospheric temperature of the drying zone 5 in the rotaryfurnace 3 that corresponds to the heat supply rate should be 800° C. orlower. Further, in order to avoid a drying time of 5 min. or longer ofthe compacts 12, a preferable atmospheric temperature is 350° C. orhigher. In this way, the atmospheric temperature of the drying zone 5 islowered for the purpose of lowering the heat supply rate. However, asthe atmospheric temperature may be relatively high, it is not necessaryto compulsorily cool the atmosphere of the drying zone 5 and the hearth6 in many cases. In those cases, an equipment configuration thatexcludes the water cooling panels 9 from the equipment configurationshown in FIG. 3 is employed. Then, the exhaust gas of incineration andreduction is prevented from flowing into the drying zone 5 and also heatis supplemented by combustion using the heating burners 11 in the dryingzone 5. It is desirable to control the heat quantity generated by theheating burners 11 to 0.2 to 0.7 times the heat quantity percircumferential length generated by the burners at the other portions ofthe rotary furnace 3.

In the case of compacts having a porosity of more than 40 to 55%, suchas the compacts produced with an extrusion forming machine or the like,as long as the heat supply rate is controlled up to 6.2 kw per one dryweight kg of compacts and also the water evaporation rate is controlledup to 2.3 g/sec. per one dry weight kg of compacts, explosion and thepowdering of a surface do not occur. In the case where such a relativelyhigh heat load is accepted, the temperatures of the atmosphere of thedrying zone 5 and the hearth 6 in the rotary furnace 3 are controlled to600° C. to 1,170° C. Here, as the atmospheric temperature is lowered bythe influence of water vapor generated from the compacts 12 or the like,as long as the temperature range is maintained, cooling with a specificdevice is not required. Inversely, strong heating may be required insome cases.

In order to satisfy the aforementioned conditions and at the same timeto control the atmospheric temperature of the drying zone 5 with highaccuracy, it is, after all, preferable to employ the equipmentconfiguration shown in FIG. 3, prevent the exhaust gas of incinerationand reduction from flowing into the drying zone 5, and install theheating burners 11 at the portion up to the vicinity of the compactfeeder 4. It is desirable to control the heat quantity generated by theheating burners 11 to 0.5 to 2 times the heat quantity percircumferential length generated by the burners at the other portions ofthe rotary furnace 3. When compacts 12 having a high porosity are driedin a rotary furnace 3 in this way, the heat transfer rate may berelatively high and a device having a simplified structure may beaccepted, and therefore the method is excellent in equipment cost andoperation cost.

The time period spent for drying compacts 12 in the drying zone 5 iscontrolled to 60 to 300 sec. When drying is finished in the short timeof 60 sec. or less, the heat amount supplied for drying the compacts 12is too much in many cases and the problem arising in the case of largecompacts 12 is that water remains in the core portions of the compacts12. On the other hand, the drying of the compacts 12 is completed within300 sec. in most cases and therefore the drying exceeding 300 sec.causes a large energy loss and a larger equipment size. For thosereasons, a preferable drying time is in the range from 60 to 300 sec. Asstated above, when various compacts 12 having different porosities aredried, the atmospheric temperature of the drying zone 5 is controlled to200° C. to 1,170° C. The atmospheric temperature is changed inaccordance with the porosity of the compacts 12.

The length of the drying zone 5 in a rotary furnace 3 is defined by thelength from the portion where raw material powder compacts 12 aresupplied to a portion 30 to 130 degrees away from the portion in therotation direction. The reasons are that: the drying time is in therange from 60 to 300 sec. and the reducing time is in the range from 8to 20 min. (480 to 1,200 sec.); it is difficult to control theatmospheric temperature of the drying zone 5 independently when thelength of the drying zone 5 is the length that corresponds to 30 degreesor less in arc angle; and others.

It is an effective means for the control of the atmospheric temperatureof the drying zone 5 to install an exhaust gas discharging flue 10 atthe boundary between the drying zone 5 and the reducing zone 7 so thatthe high temperature exhaust gas generated in the reducing zone 7 maynot flow into the drying zone 5 as stated above and shown in FIG. 3. Inthis case, when the atmospheric temperature of the drying zone 5 lowersexcessively, supplementary combustion is applied with the heatingburners 11 on the sidewalls. On the other hand, when the atmospherictemperature of the drying zone 5 is excessively high, the structure forcooling the hearth 6 and the atmosphere is required as shown in FIG. 3.As a method of cooling the hearth 6, there is the method wherein thehearth 6 is cooled by installing water cooling panels 9 on the ceilingbetween the screw discharger 8 and the compact feeder 4 as stated above.In this case, the temperature of the hearth 6 is lowered by absorbingthe radiant heat from the hearth 6 that has been exposed after thereduced compacts 13 have been discharged with metal water cooling panels9. By this method, the surface temperature of the water cooling panels 9is about 300° C. and the surface temperature of the hearth 6 is loweredto about 900° C. or lower with the cooling for 30 to 50 sec. Further, amethod of spraying water on the hearth 6 through spray nozzles or thelike at the portion upstream the compact feeder 4 is also effective forthe cooling of the hearth 6.

In this way, by the method of installing an exhaust gas discharging flue10 at the boundary between the drying zone 5 and the reducing zone 7, itbecomes possible to prevent a high temperature exhaust gas from flowinginto the drying zone 5, thus lowering the atmospheric temperature of thedrying zone 5 effectively, and to control the temperature with highaccuracy. In contrast, in the case where the atmospheric temperature ofthe drying zone 5 may be 500° C. or higher or a similar case, there isalso the method of installing a partition 14 having a gap at the lowerportion at the boundary between the drying zone 5 and the reducing zone7 as shown in FIG. 4. By the effect of the partition 14, the drying zone5 and the reducing zone 7 are separated into independent zones and itbecomes easy to control the atmospheric temperatures individually. Whenthe control of an atmospheric temperature with high accuracy is notrequired, a partition 14 may not be required and a method of controllingthe atmospheric temperature of the drying zone 5 independently from thereducing zone 7 may be employed.

Next, a typical example of equipment configuration in the case of usingan extraction forming machine, which is the most economical method as aprocess of excluding a dryer 2, is shown in FIG. 5. The methods forproducing, drying and reducing compacts with this equipment areexplained hereunder. Firstly, raw material comprising metal oxide powderin the state of containing not less than 50 mass % water and pulverizedreducing agent mainly composed of carbon is mixed and contained in amixing pit 15. As the metal oxide raw material, ore powder includingiron ore powder, manganese ore powder, chromium ore powder, etc., dustfrom a refining furnace and sludge from a rolling process, those beinggenerated in the metal producing industry, and others are used. Inparticular, sludge generated in the metal producing industry is the mostsuitable raw material because it contains about 70% water by nature.

The solid-liquid mixture of the raw material is stirred and mixed wellin the mixing pit 15. The solid-liquid mixture is transported to adehydrator 17 with a slurry transportation pump 16, dehydrated to thewater content of 15 to 27 mass % with the dehydrator, and formed intowater contained aggregates of the raw material mixture. As thedehydrator 17, a dehydrator of the type wherein solid-liquid mixture ispoured on a filter cloth that moves in a circulatory manner and squeezedwith a pair of press rolls installed above and under the filter cloth, afilter press, a centrifugal dehydrator or the like is preferably used.The produced water contained aggregates is fed to an extrusion formingmachine 18 and formed into compacts while water is contained. It ispreferable that the compacts 12 have a diameter of about 8 to 20 mm anda representative diameter of 5 to 21 mm. The compacts 12 is configuredso that water vapor may be likely to be extracted and thus the compactsmay be hardly exploded in a rotary furnace 3. Specifically, the porosityof the compacts 12 is controlled to 40 to 55%.

The compacts 12 are fed to the rotary furnace 3 in the state ofcontaining 15 to 27 mass % water. In the rotary furnace 3, the compacts12 are charged on a hearth 6 and thereafter dried in a drying zone 5while the heating rate is controlled. Specifically, the compacts 12 aredried for 60 to 300 sec. at 600° C. to 1,170° C. The compacts 12 fromwhich water has been removed (dried) in the drying zone 5 move togetherwith the hearth 6 in the furnace, are transferred to a reducing zone 7having a high temperature, are subjected to reducing reaction activelyat the time when the temperature of the compacts 12 exceeds 1,100° C.,and most of the metal oxide of the compacts 12 is transformed intometal. The reduced compacts 13 are discharged from the hearth 6 with ascrew discharger 8. The reduced compacts 13 thus produced are used asraw material for a metal reducing furnace or a metal refining furnaceincluding an electric arc furnace or a blast furnace.

EXAMPLES

The examples of the operation for drying and reducing compactscomprising the powder of metal oxide and carbon according to the presentinvention are explained hereunder. Firstly, Table 1 shows the results ofExamples 1 to 3 obtained by drying compacts in an exclusive dryer 2 andthereafter incinerating and refining them in a rotary furnace 3. Here,the treatment conditions of Examples 1 to 3 are as follows. The rawmaterial powder comprised 63 mass % iron oxide and 15 mass % carbon andthe average diameter of the powder was 11 microns. The compacts wereformed from the powder with three kinds of devices; a pan-typepelletizer, a briquette forming machine and an extraction formingmachine. The compacts produced by those methods were dried in the dryingfurnace 2 by controlling the water evaporation rate to not more than V(critical evaporation rate) and the heat supply rate to not more thanHin (critical heat supply rate). Further, the compacts after dried wereincinerated and reduced in the rotary furnace 3. In any of the reducingtreatments, the reducing time was 15 min. and the atmospherictemperature during the reducing was 1,320° C. In contrast, in the caseof Comparative Examples, the same compacts as Examples 1 to 3 were usedand subjected to incineration and reduction. However, they were dried bycontrolling the water evaporation rate to more than V (criticalevaporation rate) or the heat supply rate to more than Hin (criticalheat supply rate). The other conditions were identical to Examples 1 to3. The results are shown in Table 2.

Here, a powdering ratio was defined by the ratio of the mass (mass %) ofthe compacts less than 5 mm in size obtained by sieving the compactsafter being dried with a sieve 5 mm in mesh to the total mass of thecompacts before sieving. Likewise, a bulk yield of reduced products wasdefined by the ratio of the mass (mass %) of the compacts not less than5 mm in size obtained by sieving the compacts after reduced with a sieve5 mm in mesh to the total mass of the compacts before sieving, and aniron metallization ratio was defined by the ratio of the mass (mass %)of the metallic iron in the reduced products to the total iron mass.

TABLE 1 Example 1 Example 2 Example 3 Forming machine Pan-type BriquetteExtrusion pelletizer forming forming machine machine Compact SphericalAlmond- Columnar shaped Size *) (mm) 15 18 17 Porosity (%)   27%   33%  47% Water content (mass %) 11.5% 14.6% 21.2% Critical value of dryingV value (g/kg · sec.) 1.5 1.8 3.9 Hin value (kw/kg) 4.0 5.0 10.7 Dryingresult Actual water evaporation 0.77 1.3 2.7 rate (g/kg · sec.) Actualheat supply rate (kw/kg) 2.1 3.7 7.5 Evaluation of drying stateExistence of explosion None None None Powdering ratio (mass %)  3.9% 2.6%  3.3% Result of reduced product Iron metallization ratio (mass %)  85%   88%   88% Bulk product yield (mass %)   92%   88%   86% *) Cuberoot of compact volume

Example 1 shows the results of the operation in the case of usingcompacts a produced with a pan-type pelletizer and having a porosity of27%, which is relatively dense. The values of V and Hin computed fromthe size and the porosity of the compacts were 1.5 g/kg/sec. and 4.0kw/kg, respectively. Meanwhile, the actual water evaporation rate andheat supply rate were 0.77 g/kg/sec. and 2.1 kw/kg, respectively.Therefore, explosion was not observed since the water evaporation ratewas lower than the critical value and, further, the powdering from thecompact surfaces was as low as 3.9%. As a result of reducing thecompacts, the iron metallization ratio was as high as 85% and the bulkyield of the products was as good as 92%.

In Example 2, compacts produced with a briquette forming machine andhaving a porosity of 33% were used. The values of V and Hin computedfrom the size and the porosity of the compacts were 1.8 g/kg/sec. and5.0 kw/kg, respectively. Meanwhile, the actual water evaporation rateand heat supply rate were as low as 1.3 g/kg/sec. and 3.7 kw/kg,respectively. Therefore, explosion was not observed and further thepowdering from the compact surfaces was as low as 2.6%. As a result ofreducing the compacts, the iron metallization ratio was as high as 88%and the bulk yield of the products was as good as 88%.

In Example 3, compacts produced with an extrusion forming machine andhaving a porosity of 47%, which is a low packing density, were used. Thevalues of V and Hin computed from the size and the porosity of thecompacts were 3.9 g/kg/sec. and 10.7 kw/kg, respectively. Meanwhile, theactual water evaporation rate and heat supply rate were as low as 2.7g/kg/sec. and 7.5 kw/kg, respectively. Therefore, explosion was notobserved and further the powdering from the compact surfaces was as lowas 3.3%. As a result of reducing the compacts, the iron metallizationratio was as high as 88% and the bulk yield of the products was as goodas 86%. As it has been clarified above, as long as the drying conditionsare controlled within the ranges stipulated in the present invention,the compacts are dried well and also reduced properly.

TABLE 2 Comparative Comparative Comparative Example 1 Example 2 Example3 Forming machine Pan-type Briquette Extrusion pelletizer formingforming machine machine Compact Spherical Almond- Columnar shaped Size*) (mm) 15 18 17 Porosity (%)   27%   33%   47% Water content (mass %)11.5% 14.6% 21.2% Critical value of drying V value (g/kg · sec.) 1.5 1.83.9 Hin value (kw/kg) 4.0 5.0 10.7 Drying result Actual waterevaporation 2.5 2.2 5.5 rate (g/kg · sec.) Actual heat supply rate 5.85.9 13.9 (kw/kg) Evaluation of drying state Existence of explosionOccurred Occurred None Powdering ratio (mass %)   88%   76%   37% Resultof reduced product Iron metallization ratio Not Not 56% (mass %)operable operable Bulk product yield (mass %) — — 53% *) Cube root ofcompact volume

On the other hand, in Comparative Examples 1 to 3, the results wereobtained by drying the same compacts as Examples 1 to 3 under theconditions deviating from those stipulated in the present invention andthen reducing them. In any of Comparative Examples, as the waterevaporation rate and the heat supply rate of the compacts were largerthan the critical values respectively, the compacts were not driedproperly. In the cases of Comparative Examples 1 and 2, the compactsexploded and 76 to 88% of the compacts were decomposed into powder. As aresult, the reduction operation was not properly carried out in therotary furnace 3. Further, in Comparative Example 3, the results wereobtained by drying compacts produced with an extrusion forming machineand having a high porosity and then reducing them. In the drying of thecompacts too, the water evaporation rate and the heat supply rate of thecompacts were larger than the critical values shown by V and Hinrespectively. As a result, though explosion did not occur, 37% of thecompacts were decomposed into powder. As a result of incinerating andreducing the mixture of the bulk and powder of the compacts in therotary furnace 3, the powdered portions were influenced by thereoxidation caused by carbon dioxide gas in the atmosphere, the ironmetallization ratio was low, and the bulk yield of the products was alsolow.

Next, Table 3 shows the results of Examples 4 to 6 that are the examplesof the operations carried out by the method wherein the compacts aredried in the rotary furnace 3 as shown in FIG. 3 or 4. The treatmentconditions of Examples 4 to 6 are as follows. The raw material powdercomprised 63 mass % iron oxide and 15 mass % carbon and the averagediameter of the powder was 11 microns, which were the same as Examples 1to 3. The compacts were formed likewise from the powder with three kindsof devices; a pan-type pelletizer, a briquette forming machine and anextraction forming machine. In the drying of the compacts in thefurnace, the water evaporation rate was controlled to not more than V(critical evaporation rate) and the heat supply rate to not more thanHin (critical heat supply rate). Further, the compacts after being driedwere successively incinerated and reduced in the same furnace. Thereducing time was 13 mm. and the atmospheric temperature during thereduction was 1,300° C.

TABLE 3 Example 4 Example 5 Example 6 Forming machine Pan-type BriquetteExtrusion pelletizer forming forming machine machine Compact SphericalAlmond- Columnar shaped Size *) (mm) 17 20 20 Porosity (vol. %)   27%  33%   47% Water content (mass %) 11.5% 14.6% 21.2% Drying zone ofrotary furnace Structure of drying zone Reducing Reducing Reducing zoneand zone and zone and drying zone drying zone drying zone are are areseparated by separated separated exhaust gas by exhaust by exhaustdischarging gas gas flue, discharging discharging ceiling is flue, flue,equipped heating heating with cooling burners are burners are means.installed installed Drying zone resident 200 160 100 time (sec.) Dryingzone temperature 250–450° C. 450–750° C. 700–950° C. Critical value ofdrying V value (g/kg · sec.) 1.3 1.6 3.3 Hin value (kw/kg) 3.5 4.5 9.1Drying result Actual water evaporation 0.67 1.1 2.6 rate (g/kg · sec.)Actual heat supply rate 1.8 3.1 7.5 (kw/kg) Evaluation of drying stateExistence of explosion None None None Powdering ratio (mass %)  5.1% 5.9%  3.1% Result of reduced product Iron metallization ratio   84%  81%   83% (mass %) Bulk product yield   94%   91%   95% (mass %) *)Cube root of compact volume

Example 4 is an example of the operation in the case of using sphericalcompacts produced with a pan-type pelletizer and having a porosity of27%, which is relatively dense. The compacts have a low porosity andthey explode easily when a water evaporation rate increases. Therefore,the atmospheric temperature in the drying zone 5 was controlled in therange from the lowest temperature of 250° C. to the highest temperatureof 450° C. For that purpose, the exhaust gas discharging flue 10 wasinstalled between the drying zone 5 and the reducing zone 7 so thatexhaust gas generated in the reducing zone 7 might not flow into thedrying zone 5. Further, in order to lower the atmospheric temperatureand the hearth 6, the ceiling between the screw discharger 8 and thecompact feeder 4 at the compact supply portion and parts of the ceilingin the drying zone 5 were equipped with water cooling means. As aresult, the heat supply rate to the compacts could be reduced to 1.8kw/kg, lower than Hin, and the water evaporation rate could also bereduced to 0.67 g/kg/sec., lower than V. The reducing treatment was alsogood and the powdering ratio was as low as 5.1% and the ironmetallization ratio and the bulk product yield were high.

Example 5 is an example of the operation in the case of usingalmond-shaped compacts produced with a briquette forming machine andhaving a porosity of 33%. In this case, the heat supply rate to thecompacts was controlled to not more than Hin and the water evaporationrate was also controlled to not more than V so that the compacts mightnot cause the problems of the explosion or powdering. For this purpose,the exhaust gas discharging flue 10 was installed between the dryingzone 5 and the reducing zone 7 so that exhaust gas generated in thereducing zone 7 might not flow into the drying zone 5. Here, in the caseof these compacts, the amount of the water vapor generated from thecompacts is relatively large and, therefore, the atmospheric temperaturein the drying zone 5 lowers in excess of the target temperature in somecases. For this reason, heat was supplemented with the heating burners11 installed on the sidewalls and the atmospheric temperature wascontrolled in the range from the lowest temperature of 450° C. to thehighest temperature of 750° C. As a result, the water evaporation ratewas 1.1 g/kg/sec., lower than V. The iron metallization ratio and thebulk yield of the reduced products were good.

Example 6 is an example of the operation in the case of using columnarcompacts produced with an extrusion forming machine and having aporosity of 47%. In this case too, the heat supply rate to the compactswas controlled to not more than Hin and the water evaporation rate wasalso controlled to not more than V so that the compacts might not causethe problems of the explosion or powdering. For this purpose, theexhaust gas discharging flue 10 was installed between the drying zone 5and the reducing zone 7, similarly to Example 5. The compacts of Example6 contained a large amount of water and therefore the atmospherictemperature in the drying zone 5 dropped considerably due to the watervapor. To cope with the problem, heat was supplemented with the heatingburners 11 installed on the sidewalls and the atmospheric temperaturewas controlled in the range from the lowest temperature of 700° C. tothe highest temperature of 950° C. As a result, the water evaporationrate was 3.3 g/kg/sec., lower than V. In this operation too, the ironmetallization ratio and the bulk yield of the reduced products weregood.

Next, as Example 7, iron oxide and sludge containing carbon in aquantity generated in the processes of the steelmaking industry wereused as the raw material and formed into compacts and the compacts werereduced by using the reducing equipment shown in FIG. 5. The rawmaterial used in the operation had the average diameter of 9 microns andthe water content of 21%. The porosity of the compacts produced with theextraction forming machine was 44% and the representative diameterthereof was 15 mm. In Example 7, the temperature of the drying zone 5was controlled in the range from 890° C. to 1,020° C. and the length ofthe drying zone 5 was 150 sec. in terms of the transit time of thehearth 6. As a result of drying the compacts under those conditions, theproblems of the explosion and powdering of the compacts did not occur.The highest temperature in the reducing zone 7 was 1,300° C. and thereducing time was 13 min. The bulk yield of the reduced product ofExample 7 was as high as 91% and the iron loss to dust was as low as1.7%. Further, the iron metallization ratio was 88% and thus thereduction was also good.

INDUSTRIAL APPLICABILITY

The present invention makes it possible to dry compacts, made of powdercontaining water, properly and to reduce metal oxide economically in arotary hearth reduction process. Further, the present invention iseffective for the processing of sludge comprising metal oxide containingwater in quantity and dust containing carbon.

1. A method for drying compacts characterized by, in the event of dryingcompacts containing powder of metal oxide and carbon and also water inmass percentage by not less than 0.2 times the porosity in percentage,controlling the evaporation rate of water contained in said compacts tonot more than the value V defined below;V=300P ² /D, where, V means a critical evaporation rate of water (anevaporation rate of water per one dry mass kilogram of compacts(g/kg/sec.)), D a cube root of the volume of a compact (mm), and P aporosity (−).
 2. A method for drying compacts according to claim 1,characterized by, in the event of drying compacts having a cube root ofthe volume of 5 to 21 mm and a porosity of 22 to 32% from the statewherein 4.4 mass % or more of water is contained in said compacts,controlling the evaporation rate of water in said compacts to not morethan 0.7 g/sec. per one dry mass kilogram of compacts.
 3. A method fordrying compacts according to claim 1, characterized by, in the event ofdrying compacts having a cube root of the volume of 5 to 21 mm and aporosity of more than 32 to 40% from the state wherein 6.4 mass % ormore of water is contained in said compacts, controlling the evaporationrate of water in said compacts to not more than 1.3 g/sec. per one drymass kilogram of compacts.
 4. A method for drying compacts according toclaim 1, characterized by, in the event of drying compacts having a cuberoot of the volume of 5 to 21 mm and a porosity of more than 40 to 55%from the state wherein 8 mass % or more of water is contained in saidcompacts, controlling the evaporation rate of water in said compacts tonot more than 2.3 g/sec. per one dry mass kilogram of compacts.
 5. Amethod for drying compacts according to claim 1, characterized by usingpowder containing metallic oxide derived from a metal producing processand carbon individually or in mixture as said powder of a metal oxideand carbon.
 6. A method for reducing metal oxide characterized byincinerating and reducing compacts dried by a method according to claim5 at 1,100° C. or higher in a rotary-hearth-type furnace whereincompacts containing powder of metal oxide and carbon are loaded on theupper surface of a rotating toroidal hearth and incinerated and reducedby gas combustion heat at the upper inner space of the furnace.
 7. Amethod for reducing metal oxide characterized by, after drying compactsby a method according to claim 5 in a rotary-hearth-type furnace whereincompacts containing powder of metal oxide and carbon are loaded on theupper surface of a rotating toroidal hearth and incinerated and reducedby gas combustion heat at the upper inner space of the furnace,successively incinerating and reducing said compacts at 1,100° C. orhigher in said furnace.
 8. A method for reducing metal oxidecharacterized by, after drying compacts by a method according to claim 1in a rotary-hearth-type furnace wherein compacts containing powder ofmetal oxide and carbon are loaded on the upper surface of a rotatingtoroidal hearth and incinerated and reduced by gas combustion heat atthe upper inner space of the furnace, successively incinerating andreducing said compacts at 1,100° C. or higher in said furnace.
 9. Amethod for drying compacts characterized by, in the event of dryingcompacts containing powder of metal oxide and carbon and also water inmass percentage by not less than 0.2 times the porosity in percentage,controlling the rate of heat supply to said compacts to not more thanthe value Hin defined below;Hin=820P ² /D, where, Hin means a critical heat supply rate (a heatsupply rate per one dry mass kilogram of compacts (kw/kg)), D a cuberoot of the volume of a compact (mm), and P a porosity (−).
 10. A methodfor drying compacts according to claim 9, characterized by, in the eventof drying compacts having a cube root of the volume of 5 to 21 mm and aporosity of 22 to 32% from the state wherein 4.4 mass % or more of wateris contained in said compacts, controlling the rate of heat supply tosaid compacts to not more than 1.9 kw per one dry mass kilogram ofcompacts.
 11. A method for drying compacts according to claim 9,characterized by, in the event of drying compacts having a cube root ofthe volume of 5 to 21 mm and a porosity of more than 32 to 40% from thestate wherein 6.4 mass % or more of water is contained in said compacts,controlling the rate of heat supply to said compacts to not more than3.5 kw per one dry mass kilogram of compacts.
 12. A method for dryingcompacts according to claim 9, characterized by, in the event of dryingcompacts having a cube root of the volume of 5 to 21 mm and a porosityof more than 40 to 55% from the state wherein 8 mass % or more of wateris contained in said compacts, controlling the rate of heat supply tosaid compacts to not more than 6.2 kw per one dry mass kilogram ofcompacts.
 13. A method for reducing metal oxide characterized byincinerating and reducing compacts dried by a method according to claim1 at 1,100° C. or higher in a rotary-hearth-type furnace whereincompacts containing powder of metal oxide and carbon are loaded on theupper surface of a rotating toroidal hearth and incinerated and reducedby gas combustion heat at the upper inner space of the furnace.